Rigid thermoplastic food containers may be classified into two distinct categories. The first category of containers are the containers that the consumer acquires during the purchase of dairy or deli foods at a retail store. This category of container is often referred to as an "inexpensive" container since usually the consumer's intention is to purchase the food in the container irrespective of the type of container. Since the consumer pays the retail price necessary to purchase the food, they often think of the container as an inexpensive item which they may save for re-use at a later date.
With respect to this first category of container, a group includes the inexpensive convenience containers which consumers frequently acquire when purchasing deli foods. Generally, these "deli" containers need only to perform the function of providing a highly visible display and to provide containment of solid foods, such as, bakery items, salads or fruit at refrigeration temperature to ambient temperature. Often, the material of choice is a high clarity grade of APET (amorphous polyethylene terephthalate) or PS (polystyrene) and the containers are manufactured into many shapes by a thermoforming process so as to provide wall thicknesses of about 10-20 mils. This wall thickness range represents the low end of wall thicknesses generally seen in rigid thermoplastic food containers and as a consequence the containers can be made very inexpensively. Thus, the consumer may deem the container disposable, i.e. discardable without significant monetary loss that can be attributed to the cost of acquiring the container. In addition, the weight to volume ratio (i.e. the weight of the container over the volume of the container) of these containers is approximately 33.8-37.2 grams/liter for the total of the top and bottom of the container.
Since these deli containers have no elevated temperature requirements and thus can be formed from APET or PS, their low heat distortion temperature makes them highly unsuitable for microwave cooking applications. Furthermore, because the containers are not designed to provide a secure seal for liquid foods, the containers may leak if inverted while attempting to contain such foods during re-use by the consumer. Also, because the materials of construction are generally brittle, these containers will generally crack if stressed mechanically and thus are not very durable. Although these "deli" containers are inexpensive, their lack of heat resistance, durability and poor sealing characteristics severally limit their use.
"Dairy" containers also represent another group of thermoplastic containers that can be characterized as "inexpensive". These containers are designed to hold butters, fresh and processed cheeses, yogurts, and read-serve sauces at retail. Often these containers are filled by a food producer while the food contents are hot enough to flow freely into the container. In these cases, the materials of choice for such containers are generally PP (polypropylene) or HDPE (high density polyethylene) owing to their elevated temperature performance characteristics. The container bottoms are manufactured by an injection molding or thermoforming process. The container bottoms made by injection molding have a wall thickness of about 20-40 mils. The container bottoms made by thermoforming have a wall thickness of about 10-20 mils. Like the "deli" containers, the "dairy" containers can be made very inexpensively and as such the consumer may deem the container disposable after limited re-use. The weight to volume ratio of the injection molded containers is approximately 43.9-57.4 grams/liter for the total of the top and bottom of the container. The weight to volume ratio of the thermoformed containers is approximately 37.2 -54.1 grams/liter for the total of the top and bottom of the container.
These dairy containers almost always are formed into a round, somewhat cylindrical shape which is well suited to the requirements of high speed labeling equipment. The cylindrical shape is an inefficient shape compared to other shapes such as a rectangle or square with regard to storage space utilization.
Also the dairy containers should prevent moisture loss, oxygen permeation, and odor absorption which if not prevented would undesirably alter the food contents. The food producer solves these problems by completely enclosing the head space above the food by application of a lidding material, such as, foil or thermoplastic barrier film which is continuously sealed to the upper lip of the container bottom. The container generally will also include a removable rigid thermoplastic lid that is placed over the continuously sealed lidding foil or film so that puncture protection is provided. The lid has a wall thickness of about 10 to 20 mils. The rigid thermoplastic container lid usually engages the container bottom so as to provide a crude fit, but it does not have to provide a secure leakproof seal since the lidding material accomplishes this function. Thus when the consumer goes to re-use the container, they may find that the container's seal is unsuitable for rough transport and handling of liquid foods, since the lidding material has been peeled off during consumption of the original contents and only the crude fitting rigid lid remains.
The lid is usually made from a softer polyethylene material which often does not have the same heat resistance of the container bottom. Thus, when the consumer goes to re-use the container, they may find that the lid is easily distorted during microwave heating.
Finally, the dairy container materials are usually highly pigmented to provide opacity in order to prevent light-induced oxidation of fat-containing dairy products so as to extend store shelf-life. The consumer may prefer a see-through container which allows easy recognition of food contents during re-use of the container. Thus, dairy containers would be unsuitable in this regard. Although these dairy containers are inexpensive, their round shape, lack of a heat resistant lid, poor sealing characteristics, and opacity severely limit their re-use after the original food contents have been consumed.
For example, many consumers choose to take their lunch to work. While in transit, containers are frequently resting on uneven surfaces or being jostled from ordinary movements, and as a result, the contents of an inexpensive container may leak onto clothing, upholstery, and/or other food. In addition, the inexpensive containers may not be suitable for microwave use, and the food must be removed from the container and placed on dishware suitable for microwave cooking before being heated.
The second category of food containers consists of expensive durable containers which may utilize more expensive plastic materials and thick container walls. These containers address the shortcomings of the inexpensive containers in that they may be suitable for microwave, dishwasher, and freezer use and provide a secure seal which will not leak. However, these containers may employ costly materials. In addition, due to their sophisticated design elements, these containers generally require heavier construction to enable manufacture within the limitations associated with the injection molding process from which these containers are made. These containers have wall thicknesses of about 20 to 80 mils. Thus, this second category of containers become too expensive for disposable use. As a result, a consumer taking his or her lunch to work in an expensive durable container must be burdened with also transporting the container from work to home. The weight to volume ratio of these containers is approximately 67.6-219.6 grams/liter for the total of the top and bottom of the container.
Thus, it would be advantageous if a container were available that possessed the durability, sealing characteristics and features (i.e. microwaveable, freezable, and dishwasher safe) of the expensive containers at a cost which affords the user the option to dispose of it either after very limited use or after extended use.